Catalytic dealkylation of aromatic hydrocarbons



Patented Oct. 19, 1954 CATALYTIC DEALKYLATI ON OF ABOMATIC HYDROCARBONSHeinz Heinemann, Swarthmore, Pa., assignor to Houdry ProcessCorporation, Wilmington, Del., a corporation of Delaware No Drawing.Application October 5, 1951, Serial No. 250,017

2 Claims.

The present invention relates to an improved method for the productionof lower molecular weight aromatic hydrocarbons by dealkylation ofhigher molecular weight homologues, and is particularly concerned withthe production of lower molecular weight aromatic hydrocarbons bydealkylation of higher molecular weight homologues boiling approximatelyin the gasoline range. The invention is especially concerned with theproduction of benzene by the catalytic dealkylation of methylandethyl-substituted benzene homologues.

Among the objects of the invention is the provision of a highlyselective dealkylation process capable of producting high yields of thedesired lower molecular weight aromatic hydrocarbons while maintainingat a minimum the occurrence of accompanying side reactions which tend toproduce coke and other products at the expense of the desired aromatichydrocarbons.

In processes desribed in the art for the production of dealkylatedaromatic hydrocarbons, involving superatmospheric pressure andtemperature of about 900 to 1100 F. in the presence of added hydrogen,catalysts such as chromia and molybdena supported on carrier materialssuch as alumina and diatomaceous earth have been suggested for thedealkylation of alkyl-substituted aromatic hydrocarbons. While under theconditions advocated in these previously described processes,dealkylation of alkyl-substituted aromatic hydrocarbons present in thestock charged to dealkylation is stated to take place, side reactionsgenerally occur to a substantial extent. These side reactions involvecracking of parts of the charge and polymerization of oleflnichydrocarbons in the reaction zone. They involve, also, undesirabledisproportionation reactions. They also cause the formation ofsignificant quantities of undesirable side reaction products, includingcoke, in sufficient amounts to limit the practical on-stream operatingperiod and to require frequent regeneration of the catalyst. The extentto which such side reactions take place in these dealkylation processesis evidenced by the reported liquid product recoveries which are in theorder of about 80 to 88 percent by weight. In other related processesdescribed to involve superatmospheric pressure and temperature of about350 to 750 F. in the presence of added hydrogen, catalysts such asnickel supported on carriers such as kieselguhr have been proposed todealkylate selected alkyl-substituted aromatic hydrocarbons such asisopropylbenzene and xylenes. These processes advocated lowertemperatures and the reported yields of dealkylated product per passwere lower than in the case of the first-mentioned processes. None ofthe above-discussed processes reported the production of substantialyields of benzene from the higher benzene homologues, thus clearlyillustrating the difiiculty of selectively and completely removingmethyl and ethyl substituents from the benzene ring.

In accordance with the present invention, alkyl-substituted aromatichydrocarbons, particularly monocyclic alkyl-substituted aromatichydrocarbons, are selectively dealkylated to lower molecular weightaromatic hydrocarbons, especially benzene, under dealkylating'conditions designed to favor greatly the desired dealkylation reaction,and considerably reduce or eliminate undesirable side reactions. Thisdesirable result is achieved by carrying out the dealkylation processunder carefully chosen conditions and in the presence of particularlyselected catalysts which tend to promote dealkylation reactions but haveno significant tendency to promote undesirable side reactions either byvirtue of the catalysts inherent properties or by reason of specialcatalyst treatments described below effective in liminating or reducingsuch tendencies of the catalyst.

The supported catalysts used in the dealkylation process of the presentinvention comprise a carrier having a high surface area, generally inexcess of 50 square meters per gram (as determined by the method ofBrunnauer, Emmett and Teller described in the Journal of the AmericanChemical Society, vol. 60, pages 309 et seq. (1938)), and adapted toextensive and uniform distribution of the dehydrogenative materialthereon. Also, the carrier material used is characterized by having noappreciable activity, either alone or in composition with the minoramounts of dehydrogenative components employed, less than about 15 percent by weight of the carrier, in promotingpolymerization-depolymerization reactions of hydrocarbons underdealkylating' conditions. 7

As pointed out above, this latter characteristic of the carrier materialmay be inherent in the carrier or imparted thereto by means of a specialconditioning or inactivating treatment. An example of a preferredinherently inactive carrier material is magnesia. Titania and silica gelalso are members of this class of carrier materials. Certain naturalmaterials, such as diatomaceous earth and pumice, while inherently,relatively inert catalytically, do not provide the high surface arearequired for the dehydrogenative ma terials employed in catalysts usedin the dealkylation process of the invention and are, therefore, notrecommended for use therein.

Activated alumina is an example of a preferred carrier material of theclass requiring a special conditioning treatment to impart thereto therequired inactivity characteristic. While heat treated alumina andcommercial activated alumina are usually not regarded as crackingcatalysts in that they do not produce acceptable yields of gasoline incracking of gas oils or the like, these materials do demonstrateappreciable catalytic activity in promoting polymerization anddepolymerization of olefinic hydrocarbons and polymers, respectively.Since olefinic hydrocarbons are necessarily produced to greater or lessextent, in the various stages of the mechanisms of dehydrogenation andother hydrocarbon conversion reactions, this tendency of alumina topromote polymerization and depolymerization reactions, even thoughsmall, becomes a very significant factor from the standpoint ofintroducing undesirable side reactions. The relative importance of thisfactor is readily understood in connection with the formation of coke onthe cata lyst, which formation appears at least to considerable extentto result from the deposition on the catalyst of high molecular weightpolymers. In addition, the uncontrolled splitting of higher molecularweight olefins and polymers not directly forming coke results in sidereaction products of the type and in quantities which the practice ofthis invention avoids.

The inactivation of catalyst carrier materials, such as activated andgamma aluminas for example, is effected by treating the support with anaqueous solution of an alkaline earth compound, particularly a solutionof a magnesium or calcium salt. The treatment is generally designed tointroduce into the carrier material an amount of salt equivalent topreferably about 0.1 to 2 per cent of alkaline earth metal oxide byweight based on the carrier. The treated carrier is freed of extraneoussalts as by water washing and is then dried at about 200 F. and may becalcined, but is not heated to such high temperatures as to effecttransformation of alumina to a so-called beta form. The deactivatingeffect of this treatment is often noticeable when as little as 0.05 percent of magnesia, for example, is incorporated in an alumina carrier inthe manner described above. Amounts of incorporatedmagnesia above about2 to 3 per cent by weight of the alumina do not appear to have any addeddeactivating effect upon the alumina and may often be undesirable. Alsosuitable for deactivating the catalyst carrier material to substantiallyremove the cracking function, but not necessarily with equalquantitative effect, are the alkali metal oxides, particularly sodiumoxide, when incorporated in the alumina in a manner similar to thatdescribed in the case of the alkaline earth metal oxides, and inapproximately the same percentages.

It is preferable to incorporate the magnesia or other alkaline earth oralkali'metal compound in the carrier before incorporating thehydrogenative material, such as a metal or metal oxide. For example,catalysts prepared by treating a composite of aluminaceous carrier andhydrogenative molybdena, with magnesia in the manner described above, donot have the high degree of selectivity and completeness of dealkylationthat is observed with the preferred catalysts.

Among the catalytic dehydrogenative "materials that can be employed inthe carrier materials of the invention to produce the compositecatalysts used in the dealkylation process of the invention are themetals of group VIa of the periodic system including molybdenum andchromium oxides, and noble metals of the platinum-palladium group.

The dealkylation catalysts containing dehydrogenative materials, such asmolybdena. (molybdenum oxide) supported on a carrier, can be prepared bydipping the inactive or inactivated carrier material into a solution ofa soluble molybdenum compound, such as ammonium molybdate, to cause someof the solution to be sorbed by the carrier, and then drying anddecomposing the molybdenum compound to molybdena. The amount of molybenathus incorporated in the carrier material should generally be less thanabout 15 per cent, and preferably less than about 12 per cent by weightbased on the final catalyst composite.

In the case of the noble. metal catalysts, the platinum, or other noblemetal component, is generally employed in the catalyst in amounts lessthan about 2 percent by weight of the carrier (water free basis), andpreferably in the range of about 0.05 to 1 percent by weight of thecarrier. The metal can be added to the carrier by impregnating theinactive or inactivated carrier with an aqueous suspension of the metaloxide and then reducing the oxide to the free metal; or by dipping thecarrier into a solution of a soluble compound of the metal, and thendrying and decomposing the metal compoundtaken up by the carrier to thefree metal or oxide. In the latter case the oxide is, of course,subsequently reduced to the free'metal.

The conditions selected for the specific dealkylation reactionresultingin the production of benzene and lesser amounts of lowerbenzene homologues include temperatures preferably not below about 800to 900 F. and as high as 1000 F., or higher. Temperatures below about800 F. are not recommended because at these lower temperatureshydrogenation of aromatic hydrocarbons to naphthene hydrocarbonstends-to occur with consequent reduction in yield of desired aromatics.The selected dealkylation conditions also include pressures preferablynot above about-.1000 pounds per square inch and as low as about poundsper square inch, or lower. Pressures above 1000 pounds per square inchmay be used but better results are obtained at the lower pressures. Moreparticularly, the conditions for the more specific and relatively 'morecomplete removal of methyl or ethyl substituents from the benzene ringinclude temperatures in the range of about 900 to 1000 F., pressures inthe range of about 100 to 800 pounds per square'inch, space rates ofabout 3 (liquid vols.- oil/hn/vol. cat.) or less, preferably 2 or less,and hydrogen to on ratios in the range of about '3 to -10 moles hydrogenper mole of oil, preferably a ratio of about 4 to 1 moles hydrogen permole aromatic hydrocarbon charge.

The dealkylation process of the invention i is particularly designed toremove'more'compl'etely and more selectively alkyl substituents fromalkyl-substituted aromatic hydrocarbons than in the case of previousknown dealkylation processes discussed above, and is particularlyadapted to dealkylate alkyl-substituted monocyclic aromatichydrocarbons, especially methyland/or ethylsubstituted benzenehydrocarbons, and more especially toluene and xylenes. The demeth'an- '5ation of toluene and xylene charge stocks takes place with relativelyhigh yields per pass, with negligible coke or other by-productformation, and with exceptionally high liquid product recoveries. As aresult of these unique features of the process, the dealkylated portion,e. g. benzene, of the recovered liquid product is separated from theundealkylated portion of the liquid product by a relatively simpleoperation, such as fractional distillation, and the undealkylatedportion can be recycled repeatedly through the dealkylation stage of theprocess together with added fresh charge. Since negligible cokeformation takes place in the process, the frequent catalyst regenerationrequired by previous dealkylation,

processes is eliminated, and on-stream periods of several weeks or moreare possible. Such operation by virtue of the high selectivity and highdegree of completeness of the present dealkylation process results inunusually high yields of the desired low molecular weight aromatichydrocarbons.

The instant dealkylation process is especially adapted to the treatmentof various narrow boiling hydrocarbon fractions rich in aromatichydrocarbons boiling in the toluene and xylene range. The treatment ofthese narrow fractions, which will also include ethyl benzene, by theinstant process, by virtue of the advantages and features pointed outabove, results in the production of unusually high yields ofdemethanated aromatic hydrocarbons, and especially good yields ofbenzene. The production of benzene in good yields from toluene is, ofcourse, a much more difiicult operation than that of the production oftoluene from xylenes. The success of the instant process in the moredifficult operation of demethanation of toluene is a measure of thegreater success of the process of the invention in the demethanation orthe dealkylation of other methylated or alkylated aromatic hydrocarbons,respectively.

The present process is also applicable to greater advantage, for thereasons given above, to the dealkylation of alkyl-substituted aromatichydrocarbons contained in various naphtha charge stocks such as straightrun gasoline, catalytically cracked distillate, distillates from thermalcracking, and the like, as well as products and fractions fromhydrogenative reactions.

The following example illustrates the method of preparation of a typicalcatalyst, and the method of use of the catalyst in the dealkylationprocess of the invention.

Example Commercial activated alumina pellets (2.59 kg.) were treatedwith a 20 percent magnesium chloride hexahydrate solution by soaking thepellets in about 4 kg. of the solution for about 30 minutes. During thesoaking period the pellets were stirred several times and the pH of thesolution changed from an initial value of 5.8 to a final value of 7.0.The soaked pellets were washed free of chlorides and were dried overnight at 200 F. The dried product was dipped for about 30 minutes in asolution of ammonium molybdate containing about 1.12 kg. of ammoniummolybdate in 3.5 liters of solution. The treated product was drained anddried for 3 hours at 200 F., and finally heat treated at 1050 F. for 2hours. Before being placed on stream, hydrogen was passed over thecatalyst to reduce the molybdenum to lower valence forms. The finalcatalyst contained an analysis, approximately Run Number 1 2 3 Pressure,p. s. i 800 300 Liquid Space Velocity, V./V./Hr 2 2 1 Liquid Recovery,Vol. Percent.. 98.4 97.4 93. 6 Liquid Recovery, Wt. Percent 97. 9 95. 393. 5 Benzene Yield, Wt. Percent of Feed 7. 9 10.9 11. 3 TolueneDemethanation, Wt. Percent. 9. 3 12. 8 l3. 3

The data indicate that dealkylation proceeds more satisfactorily atmoderate pressures in the range of about 100 to 500 pounds per squareinch than at higher pressures in the range of about 500 to 1000 poundsper square inch. The amount of coke and other side reaction productsproduced in these runs was negligible. Thus substantially all of thetoluene feed converted by the reaction was demethanated to producebenzene in contradistinction to previous known dealkylation processes inwhich appreciable cracking takes place and relatively lower liquidrecoveries are obtained due to the production of appreciable amounts ofcoke and cracked gases.

Similar results as to desired dealkylation selectivity and desireddealkylation completeness, resulting in high yields of the desireddealkylated aromatic hydrocarbon, are obtained when using noble metalsof the platinum-palladium group as the dehydrogenative component on aninactive or inactivated catalyst carrier material; and also whencharging the various other methyl-, ethyland alkyl-substituted aromatichydrocarboncontaining fractions and materials described above, as thefeed materials in the described dealkylation process of the invention.

Obviously many modifications and variations of the invention ashereinbefore set forth may be made without departing from the spirit andscope thereof and therefore only such limitations should be imposed asare indicated in the appended claims.

I claim as my invention:

1. The method of producing benzene comp-rising contacting a charge stockcontaining toluene in the presence of added hydrogen with a catalystcomprising an aluminaceous carrier of attenuated cracking activity andcontaining less than 15% (determined as M003) by weight of the catalystof molybdenum oxide, said catalyst being one prepared by impregnating anactivated alumina with magnesia and calcining the impregnated alumina atelevated temperature below that effecting transformation to beta formfollowed by incorporation of the molybdenum oxide; said contacting beingcarried out at a temperature in the range of about 900 to 1000 F., apressure in the range of about 100 to 800 pounds per square inch, ahydrogen to toluene ratio of about 4 moles hydrogen per mole toluene anda space rate in the range of about 1 to 2 liquid volumes of toluene perhour per volume of catalyst.

2. The method of producing benzene which comprises contacting a chargestock comprising methyl substituted benzene hydrocarbons under asuperatmospheric partial pressure of hydrogen with a catalyst composedof activated alumina of attenuated cracking activity containing a minorquantity of a dehydrogenating component, said contacting being performedunder conditions favoring dealkylation of methyl substitutedhydrocarbons in the charge stock and including pressure in the range ofabout 100-800 pounds per square inch and temperature in the range ofabout 900-1000 F., and recovering abenzene fraction from the product;said catalyst being prepared by impregnating an activated alumina with0.1 to about 2% by weight of magnesia and calcining the impregnatedalumina at elevated temperature below that effecting transformation tobeta form, and thereafter incorporating in the magnesia-impregnatedalumina up to 12% by weight (determined as M003) of molybdenum oxidebased on the final catalyst composite.

References Cited in the iileof this patent UNITED STATES PATENTS OTHERREFERENCES Ipatieff, Jour. Amer. Chem. Soc., vol. 55, pp. 3696-3701(Sept. 1933) (6 pages).

1. THE METHOD OF PRODUCING BENZENE COMPRISING CONTACTING A CHARGE STOCKCONTAINING TOLUENE IN THE PRESENCE OF ADDED HYDROGEN WITH A CATALYSTCOMPRISING AN ALUMINACEOUS CARRIER OF ATTENUATED CRACKING ACTIVITY ANDCONTAINING LESS THAN 15% (DETERMINED AS MOO3) BY WEIGHT OF THE CATALYSTOF MOLYBDENUM OXIDE, SAID CATALYST BEING ONE PREPARED BY IMPREGNATING ANACTIVATED ALUMINA WITH MAGNESIA AND CALCINING THE IMPREGNATED ALUMINA ATELEVATED TEMPERATURE BELOW THAT EFFECTING TRANSFORMATION TO "BETA" FORMFOLLOWED BY INCORPORATION OF THE MOLYBDENUM OXIDE; SAID CONTACTING BEINGCARRIED OUT AT A TEMPERATURE IN THE RANGE OF ABOUT 900 TO 1000* F., APRESSURE IN THE RANGE OF ABOUT 100 TO 800 POUNDS PER SQUARE INCH, AHYDROGEN TO TOLUENE RATIO OF ABOUT 4 MOLES HYDROGEN PER MOLE TOLUENE ANDA SPACE RATE IN THE RANGE OF ABOUT 1 TO 2 LIQUID VOLUMES OF TOLUENE PERHOUR PER VOLUME OF CATALYST.